Field of the Invention
The present invention relates to a method of injection molding a composite material part, in particular in the field of aviation.
Description of the Related Art
Such a method generally consists in placing a preform in a mold, the preform being made by three dimensionally weaving yarns, for example, in injecting a resin into the mold so as to impregnate the preform, and then in unmolding the part after the resin has cured. Such a method is known as resin transfer molding (RTM).
Patent application FR2950286, in the name of the applicant, discloses a method of the RTM type for fabricating a composite material blade, the method consisting in:                making a preform by three dimensionally weaving yarns, the preform comprising a root portion and an airfoil portion that are connected together by sloping side faces that are to form bearing surfaces for the root in an assembly slot in a rotor disk;        compacting the preform in a mold having a recess that includes first sloping side faces corresponding to the above-mentioned side faces of the preform, and second sloping faces that bear against the root portion of the preform, and that slope in the opposite direction to the first sloping side faces; and        injecting a resin under a vacuum into the mold and polymerizing the resin.        
The fiber preform is thus embedded in a rigid resin matrix, the fibers of the preform having a mechanical reinforcement function, and the resin matrix serving to hold the part together.
Depending on the injection parameters and on the type of resin used, pores may appear in the injected part, in the core of the part, and/or in its surface. These pores weaken the part considerably, and the part must then be scrapped.
Studies have shown that these pores stem from a chemical compound that is generated during curing of the resin, which compound is present in gaseous form under the temperature and pressure conditions inside the mold.
The gas creates bubbles that are held captive in the preform and in the resin, and on hardening during curing, they produce pores.